Documentation

regChars returns a filter function that tells if a char is a member
of the regChar expression or not. The regChar expression is basically a
set of chars, but it can contain ranges with use of the - (dash), and
it can also be specified as a complement set by prefixing with ^
(caret). The dash and caret, as well as the backslash, can all be
escaped with a backslash to suppress their special meaning.

NOTE: The . (dot) is allowed to be escaped. It has no special meaning
if it is not escaped, but the default filename_toks in
Darcs.Commands.Replace uses an escaped dot (WHY?).